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United States Patent |
5,253,524
|
Abbink
,   et al.
|
October 19, 1993
|
Integrated accelerometer with coil interface spacer
Abstract
An integrated accelerometer includes features for minimizing the effects of
foreseeable operational stresses. The accelerometer is of a type that
includes a planar support base having an internal aperture for
accommodating a hinged strut and limit stops for minimizing the travel of
the shadow paddle portion of the strut. The limit stops are fabricated of
material of suitable spring constant to provide an appropriate degree of
"give" without degrading instrument performance or accuracy. Two-piece
arrangements sandwich the hinges, thereby limiting flexure to an
acceptable range. The predictable mechanical stresses resulting, for
example, from thermal coefficient mismatches between the materials of the
torquer coil and the strut are minimized by the use of a spacer
intermediate the base and the coil. The spacer, fabricated of material
whose thermal coefficient closely matches that of the pendulous mass is
generally circular, and includes diametrically-opposed prongs on the upper
and lower edges thereof, such arrangements of prongs being staggered by
ninety degrees.
Inventors:
|
Abbink; Henry C. (Westlake, CA);
Sakaida; Daryl K. (Northridge, CA);
Wyse; Stanley F. (Encino, CA)
|
Assignee:
|
Litton Systems, Inc. (Beverly Hills, CA)
|
Appl. No.:
|
951799 |
Filed:
|
September 28, 1992 |
Current U.S. Class: |
73/497; 73/514.36 |
Intern'l Class: |
G01P 015/13 |
Field of Search: |
73/497,517 B,862.61
|
References Cited
U.S. Patent Documents
4697455 | Oct., 1987 | Norling | 73/497.
|
4788864 | Dec., 1988 | Pier | 73/517.
|
Primary Examiner: Chapman; John E.
Attorney, Agent or Firm: Kramsky; Elliott N.
Parent Case Text
This application is a continuation of application Ser. No. 633,260, filed
Dec. 24, 1990, now U.S. Pat. No. 5,191,794.
Claims
What is claimed is:
1. An integrated accelerometer comprising in combination:
a) a planar support base having an internal aperture;
b) a pendulous mass, said mass including a pair of arms;
c) a pair of spaced flexible hinges for pivotally supporting said pendulous
mass within said aperture;
d) a substantially round torquer coil;
e) a substantially round interface spacer of material having a coefficient
of thermal expansion that is less than that of the coil, said spacer being
fixed to said pendulous mass and to said coil for substantially limiting
the transmission of mechanical stresses between said coil and said
pendulous mass; and
f) said spacer including (i) a pair of upstanding prongs at the upper edge
of said spacer for contacting said torquer coil and (ii) a pair of
downwardly-directed prongs at the lower edge of said spacer for contacting
said pendulous mass.
2. An integrated accelerometer as defined in claim 1 wherein said interface
spacer is further characterized in that;
a) said upstanding prongs are located at substantially
diametrically-opposed portions of said upper edge of said spacer; and
b) said downwardly-directed prongs are located at substantially
diametrically-opposed portions of said lower edge of said spacer.
3. An integrated accelerometer as defined in claim 2 wherein said pair of
upstanding prongs is located at said upper edge of said spacer
substantially orthogonal with respect to the location of said
downwardly-directed prongs at the lower edge of said spacer.
Description
BACKGROUND
Related Application
The present invention is related to U.S. patent application Ser. No.
121,088 of Henry C. Abbink and Nicholas F. Pier entitled "Integrated
Accelerometer Assembly" which was filed on Nov. 16, 1987, now U.S. Pat.
No. 4,987,780.
Field of the Invention
The present invention relates to integrated accelerometers. More
particularly, this invention pertains to an integrated accelerometer that
includes a number of improved sub-assemblies for effecting enhanced
accuracy and durability.
Description of the Prior Art
Accelerometers of the hinged, pendulous mass type have substantially
replaced floated accelerometers in modern strapdown inertial navigation
systems. Such hinged accelerometers offer smaller size, lighter weight and
simplified construction. Functionally, a common type of single axis
accelerometer comprises a pendulous mass that is suspended within a
housing by flexure type hinges. When subjected to acceleration, the mass
pivots or rotates about the hinge axis to thereby shutter the output of a
light-emitting diode (LED) that is conventionally located adjacent to the
mass' non-pivotal or "free" end. This is detected by a photodetector
circuit. The circuit produces an output signal that is proportional to the
sensed acceleration. Such signal is amplified and the resultant current is
applied to a torquer coil that is mounted to the pendulous mass. The coil
reacts with a permanent magnet that is affixed to the housing to return
the mass to a substantially neutral (null) position. The torquing current
provides a measure of the input acceleration.
While the foregoing arrangement may provide excellent performance in a
relatively small package, its manufacture requires intensive manual
assembly and adjustment resulting in substantial expense. Conventional
designs include minute components that are attached by Epoxy or solder in
processes that require highly dexterous and skilled personnel.
SUMMARY
The foregoing and additional shortcomings of the prior art are addressed by
the present invention that provides an integrated accelerometer with
durable sub-assemblies.
The accelerometer includes a planar support base having an internal
aperture. A pendulous mass includes a pair of spaced flexible hinges for
pivotally supporting the pendulous mass within the aperture. A torquer
coil is substantially round. A substantially round interface spacer of
material having a coefficient of thermal expansion that is less than that
of the coil, is fixed to the pendulous mass and to the coil to limit the
transmission of mechanical stresses between the coil and the pendulous
mass substantially. The spacer includes a pair of upstanding prongs at its
upper edge for contacting the torquer coil and a pair of
downwardly-directed prongs at the lower edge for contacting the pendulous
mass.
The preceding and additional aspects of the present invention shall become
further apparent from the detailed description that follows. This detailed
written description is accompanied by a set of drawing figures. Numerals
of the drawing figures, corresponding to those of the written description,
point to the various features of the invention. Like numerals refer to
like features throughout both the drawing figures and the written
description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an integrated accelerometer in accordance
with the invention;
FIG. 2 is an exploded perspective view of the integrated accelerometer of
the invention;
FIG. 3 is bottom plan view of the integrated accelerometer of the
invention;
FIG. 4 is a top plan view of the integrated accelerometer of the invention;
FIG. 5 is a cross-sectional view taken at line 5--5 of FIG. 3 for
illustrating the arrangement of the invention for attaching compliant
limit stops to the planar base of the integrated accelerometer of the
invention;
FIG. 6 is a cross-sectional view taken at line 6--6 of FIG. 3 for
illustrating the hard hinge stop of the integrated accelerometer of the
invention; and
FIG. 7 is a cross-sectional view taken at line 7--7 of FIG. 3 for
illustrating the torquer coil arrangement of the ceramic accelerometer of
the invention.
DETAILED DESCRIPTION
FIG. 1 is a perspective view of a ceramic accelerometer in accordance with
the invention. The accelerometer 1? comprises a compact, simplified and
relatively easy-to-manufacture device that is suitable for batch
processing. This is to be contrasted with the complex and accordingly
difficult-to-manufacture assemblages of the prior art that include
block-like metallic support frames and require numerous discrete fasteners
that demand hand-assembly. An improvement over this type of accelerometer
is disclosed and taught in co-pending U.S. patent application Serial No.
121,088 of Abbink et al. entitled "Integrated Accelerometer Assembly". The
device disclosed in that 15 patent application represents an improvement
over the bulky assemblages of the prior art and, due to its
generally-planar structure and the types of materials and modes of
fixation of sub-assemblies employed, is amenable to economical
manufacture. (Note: Although the element 10 is termed an "accelerometer"
throughout this application, it is understood by those in the art that the
disclosed device does not constitute a complete accelerometer which would,
of course, also include a magnet and a magnetic circuit in addition to the
component elements enumerated herein.)
The present invention represents a further refinement in integrated
accelerometer design by providing a planar integrated accelerometer 10 of
increased durability, accuracy and useful life. More specifically, the
present invention comprises an accelerometer 10 that includes a number of
improved sub-assemblies that interact to enhance the basic planar
integrated accelerometer concept. Such sub-assemblies include a limit stop
12, a spacer 14 for a mechanically isolating the accelerometer torquer
coil 16 from the strut 28 to overcome the effects of unavoidable
thermal-mechanical mismatches and hard hinge stop assemblies 20 and 22
that limit the flexure of hinges 24 and 26 within "safe" ranges.
FIG. 2 is an exploded perspective view of the accelerometer 10. The
operation and general arrangement of such apparatus is well-known. A strut
28 that includes legs 30 and 32 is flexibly positioned within an internal
aperture 34 of the accelerometer base 18 by means of hinges 24 and 26. The
base 18 is generally planar. A shadow paddle 36 is fixed to the free end
of the strut 28.
A light-emitting diode (LED) 38 is accommodated within a recess 42 in the
internal aperture 34 and thereby fixed to the base 18. A photodetector 40
is similarly fixed to the base 18 in opposed relationship to the LED 38.
When assembled, the shadow paddle 36 lies between the LED 38 and the
photodetector 40 when the pendulous mass 28 is unstressed. That is, the
hinges 24 and 26 maintain the attitude of the strut 28 relative to the
base 18 absent any external acceleration force, so that the shadow paddle
36 interrupt the transmission of emitted light from the LED 38 to the
photodetector 40. The imposition of an acceleration force normal to the
plane of the base 18 (i.e. the sensitive axis of the device) deflects of
the free end of the pendulous mass 28 relative to the base 18 and causes a
corresponding displacement of the shadow paddle 36. Light, proportional in
amount to the physical degree of displacement of the shadow paddle 36,
will be detected at the photodetector 40 and a d.c. electrical signal of
corresponding value thereby generated.
The d.c. signal is transmitted to a conventional feedback control system
(not shown) wherein a corrective current is generated that is delivered to
the torquer coil 16 to drive the accelerometer magnetically in a direction
opposite to the displacement caused by the acceleration force. The amount
of current required to return the shadow paddle 36 to "neutral" (as
indicated by the photodetector 40) provides a measure of the acceleration
force sensed.
Metal conductors 44 and 46 are formed upon the legs 32 and 30 respectively
of the strut 28. Since the metallized conductors and the strut members are
of different thermal expansion coefficients, undesirable bending that
could produce inaccurate measurements is avoided by plating similar
materials to both the top and bottom surfaces of the legs. As can be seen
in FIG. 4, a top plan view of the accelerometer 10, metal conductors 46'
and 44' of corresponding design are fixed to the opposed sides of the legs
30 and 32 respectively. The wire conductors 48 and 50 that provide
electrical communication between the conductors 44' and 46' and the
torquer coil 16 can be clearly seen in FIGS. 3 and 4, bottom and top plan
views of the accelerometer respectively.
Returning to FIG. 2, the limit stops 12 and 12' are substantially mutually
aligned. Pads 52, 54, 56 and 58 of residual metal cover laser weld
"anchors" (discussed below) fabricated of layers of chrome, nickel and
gold. Spacers 60, 62 are located between the ends of the limit stop 12 and
the pads 52 and 54. Each of the spacers is preferably fabricated of 0.001
inch thick photoformed Elgiloy. Pairs of leaves 64, 66, and 68, 70
comprise hard hinge stop assemblies 20 and 22 respectively. The leaves 64
through 70 are preferably of 0.005 inch stainless steel. They may be
alternatively formed of 0.003 inch thick photoformed Elgiloy. As will be
discussed in greater detail below, the hinges 24 and 26 are attached to
the base 18 by laser welds (alternatively, by Epoxy or like adhesive)
while the leaf-like hard stops comprised of the leaves 64 through 70 are
laser welded thereto.
FIGS. 3 and 4 are bottom and top plan views of the assembled accelerometer
10. Primed numerals indicate features fixed to the top of the
accelerometer 10 that correspond in structure to like features identified
previously with reference to the bottom of the accelerometer. In FIG. 4,
it can be noted that metallizations 72 and 74, in electrical contact with
wires 76 and 78, are provided for electrical communication between the LED
38 and the feedback control circuitry (not shown).
As can be seen in FIGS. 3 and 4, limit stops 12 and 12' are positioned at
opposite sides of the accelerometer base 18 for limiting the deflection of
the strut 28 in response to an input acceleration force along its
sensitive axis. The limit stops 12 and 12' are of a compliant or "springy"
design and fabrication. Unlike arrangements of the prior art which do not
provide an appreciable amount of "give", the limit stops 12 and 12' act
like shock absorbers, reducing hinge stress that can produce undesirable
bias shifts when a hinge is bent (i.e. deformed due to shock). Due to the
design and material compositions of the stops 12 and 12', they may be
temporarily deformed within elastic limits and flexed to facilitate the
removal of grit and the like. Furthermore, the stops 12 and 12' can be
adjusted by placing a shim thereunder to force a wider gap if it is
desired to permit increased deflection of the strut 28.
The limit stops 12 and 12' are laser welded at opposed ends 80 and 82 (80'
and 82') to laser weld anchors in the accelerometer base 18. Each limit
stop is of a roughly U-shaped design including inclined side arms 84 and
86 joined to a central arm 88 for contacting the mass 28. The limit stops
12 and 12' are proportioned so that the central arms 88 and 88'
approximately overlie and are positioned to contact the pendulous mass 28
at its center of percussion. This prevents the imposition of translational
stresses on the hinges 24 and 26 to enhance the durability and accuracy of
the accelerometer 10.
FIG. 5 is an enlarged cross-sectional view of the accelerometer taken at
line 5--5 of FIG. 3 that illustrates the means for anchoring the ends of
the limit stops 12 and 12' to the base 18. As can be seen, the ends 80 and
82 (80' and 82') of the limit stops are laser welded to regions 90 and 92
that have been electroplated with copper. Laser welded nuggets 94 and 96
secure the 0.003 inch thick photoformed limit stops 12 and 12' and 0.001
inch thick spacers 60 and 62 of Elgiloy in sandwich-like arrangements as
shown. The nuggets substantially comprise mixtures of Elgiloy and copper
with traces of chromium, nickel and gold as a result of the laser welding
process.
FIG. 6 is a cross-sectional view of the accelerometer taken at line 6--6 of
FIG. 3 that illustrates the arrangement of a representative hard hinge
stop 20 of the invention. The hinge stops act in cooperation with the
compliant limit stops to protect the hinges from the potentially
deleterious and deforming effects of both extreme acceleration forces and
transients in the signals applied to the coil 16 that act upon the strut
28.
The hinge 24 is fixed to the base 18 by laser welding to copper anchors or
by an appropriate adhesive such as Epoxy. A notch 98 in the hinge 24
defines the region of maximum flexure. Leaves 64 and 66 of the hard hinge
stop 20 are arranged immediately above and below the hinge 24 and in
contact therewith. The leaves 64 and 66 (of 0.003 inch thick photoformed
Elgiloy) are individually fixed to the top and bottom of the hinge 24 by
means of laser welds 100 and 102 respectively. Referring to both FIG. 6
and the prior figures, the leaves are seen to comprise matching shapes
that extend beyond the notch 98 and thereby encompass the region of
maximum hinge flexure. The substantially pear-shaped leaves terminate in
transverse lips 104 and 106 that extend across the width of the hinge 24
beyond the flexure-defining notch 98 and provide a means for restraining
hinge flexure in a direction of translation perpendicular to the
accelerometer base 18. Due to the relative location of the lips 104, 106,
the hard stops permit rotation of the strut 28 well beyond the limit
established for translation of the hinges.
FIG. 7 is a cross-sectional view of the accelerometer 10 taken at line 7--7
of FIG. 3 that provides a detailed view of the arrangement and interaction
that exists between the coil spacer 14 and the torquer coil 16. In the
prior art, the coil 16, of conductive metallic composition such as copper,
is fixed to the strut 28. Due to the existence of significant mismatches
between the thermal coefficients of expansion of the material of the strut
28 and that of the torquer coil 16, harmful, perhaps delaminating, strains
can occur at the coil-to-mass interface. This is avoided in the present
invention by the use of a spacer 14 of unique design. The spacer 14 is
formed of material of compatible thermal expansion coefficient to that of
the strut 28. Thus, changes in temperature do not induce undesirable
stresses at the interface between the spacer 14 and the strut 28.
Furthermore, the spacer includes upper and lower pairs of
diagonally-opposed prongs 108, 110, 112 and 114. The upper pair of prongs
112, 114 contacts the thermally-incompatible torquer coil 16. Thus,
differential expansions and contractions between the coil 16 and the
spacer 14 are limited in effect to the inward and outward deflections of
the upper prongs 112 and 114 that are translated into corresponding
bending of the torquer coil 16 to assume an elliptical shape as permitted
by the prong structure. The bending of the coil 16 effectively absorbs the
stresses that would otherwise be transmitted to the strut 28.
The placement of the lower prongs 108 and 110 at a ninety degree
displacement from the upper pair of prongs isolates the lower pair from
the effect of thermal incompatibility between the upper portion of the
spacer 14 and the torquer coil 16.
Thus, it is seen that the present invention provides an improved integrated
accelerometer that incorporates many features which enhance reliability,
effective lifetime and accuracy. By utilizing the teachings of this
invention, one may enjoy the inherent manufacturing advantages of a planar
integrated accelerometer while achieving enhanced performance.
While this invention has been disclosed with reference to a presently
preferred embodiment, it is not limited thereto. Rather, the scope of this
invention is limited only insofar as defined by the following set of
claims and includes all equivalents thereof.
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